Rescale internal Slider values to avoid integer overflow

Bart De Vries requested to merge bdevries/elisa:work/rescale-slider into master

The original implementation used the raw duration and position as reported by qmultimedia, which are in milliseconds. This, combined with the actual Slider implementation in qqc2-desktop-style caused value/duration to overflow at about 3 minutes and 30 seconds. The adapted implementation uses seconds for the internal Slider values.

CCBUG: 455339

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